Indigo has been used to dye fabric with "indigo blue" since before recorded history. The sap which oozes from the plant when bruised was applied to fabric by ancient Egyptians, Greeks and Romans. Indigo has been used in India to dye fabric for at least 4,000 years by methods which are practically identical to methods employed today. Indigo was introduced in Europe in large quantities by the Dutch East India Company in the early 17th century.
Indigotin (C.sub.16 H.sub.10 N.sub.2 O.sub.2) is the true coloring matter of indigo. When pure, indigotin forms a dark, rich blue powder or bronzy blue-colored needle crystals. The most important reaction of indigotin is its reaction with reducing agents. When subjected to a reducing agent in the presence of alkali, indigotin combines with two atoms of hydrogen and is reduced to a colorless body, known as indigo-white or the leuco form, which is insoluble in water, but dissolves in alkali, with a yellow color. This reaction may be represented, as follows: ##EQU1##
It is reoxidized to indigotin with great ease, simply by exposure to oxygen, by the following equation: EQU C.sub.16 H.sub.12 N.sub.2 O.sub.2 +O=C.sub.16 H.sub.10 N.sub.2 O.sub.2 +H.sub.2 O
The commercial vat method utilizes these reactions to dye fabric or yarn indigo blue. Commercial indigo vat dyeing is carried out in an aqueous alkali vat containing the reduced leuco form of indigo. Fibers in the form of warp yarns are dipped into the vat for a residence time sufficient to permit the fibers to absorb the desired amount of leuco dye. Following each dip, the fibers are squeezed between rolls and then carried into the open air, which is known as "skying," whereby the leuco dye is oxidized to the insoluble indigotin form in the fibers.
Oxidation of the reduced leuco form in the dip vat is troublesome in vat dyeing. The oxidation occurs where the dye liquor contacts the air, especially in the region of the squeeze rolls where multiple interfaces are generated, exposing large areas of leuco dye liquor to the air as the dye liquor drains from the squeeze rollers. This oxidation results in significant loss of dye liquor and formation of a floating scum of oxidized insoluble dye. U.S. Pat. No. 4,283,198 discloses an indigo dye process which includes an enclosure of the air-liquor interface of a leuco indigo bath comprising sodium hydrosulfite and sodium hydroxide with floating planks of foamed plastic, and with a box-like container covering the fabric on a portion of its upward travel into a set of rollers. This enclosure results in consumption of oxygen at the enclosed interface by reducing agents in the dye liquor. However, the enclosure cannot be substantially gas tight and would not result in an inert gas-enclosed process. Further, the process is impractical for commercial indigo dyeing operations.
Yoshii, A., et al. (1987), "Studies on the Dyeing of Indigo II--The Dyeing Behavior of Indigo Pure in Nitrogen," Sanyo Gakueb Tanki Daigaku Kenkyu Ronshu (Assortment of Research Papers at Sanyo Gakuen Junior College), 18:55-62, discloses bubbling nitrogen through an Erlenmeyer flask containing leuco indigo and reducing agents. This reference discloses satisfactory dyeing of test squares of fabric which are released into the dye liquor from a compartment in the laboratory apparatus after the nitrogen has been bubbled in. This article does not disclose a method for prevention of formation of oxidized indigo scum in conventional indigo dye equipment, nor any commercially viable method of contacting the fabric with the dye liquor. Further, this article does not teach that the amount of reducing agent required is less in the presence of nitrogen.
Synthetic indigo has essentially replaced natural indigo in commercial dyeing. The production of synthetic indigo requires the use of toxic materials, including sodium cyanide, formaldehyde, sodium and potassium hydroxide and synthetically produced aniline, which are carried through to the cloth. The waste products include the unspent toxic chemicals described above, plus sulfuric and hydrochloric acid, sodium hydroxide and insoluble salts. Thus, there has been a renewed interest in the use of natural indigo; however, the prior natural indigo dye methods result in dyed fibers or fabric which are not nearly as washfast and lightfast as fibers dyed with synthetic indigo dyes, and naturally dyed colors are difficult to reproduce using known techniques.
Numerous methods are known to the art for dyeing fibers and fabrics. In the production of a textile product the dyeing step can be accomplished at any one of several points along the way. Generally speaking, all of the dye processes can be broken down into two categories, batch processes and continuous processes. Batch processes involve the immersion of the fiber in a dyebath for extended periods of time, whereas in continuous processes the fibers pass quickly through the dye. Additionally, dye processes can be broken down into two other categories, atmospheric and pressure. This categorization refers to whether or not the dyeing is performed under pressure.
In the earliest stages of making a textile product, the raw fiber can be dyed prior to being spun into a yarn. This is known as stock dyeing and is used to create special effects, such as heathered yarns, by blending different colors or by blending a dyed yarn with an undyed yarn. The different colors of stock can be blended and spun together to create various effects. This method is used for fibers such as cotton and wool. The saying "dyed in the wool" apparently refers to stock dyeing. Stock dyeing is a batch process that can be atmospheric or pressurized.
There are two primary methods for dyeing yarn. The first method is known as skein dyeing. A skein of yarn is similar to a coiled rope. This method is used primarily for dyeing silk and heavy wool yarns, though some cotton is dyed this way also. Skein dyeing can be performed in pots on gas burners or in more specialized machinery. Skein dyeing is a batch process and is generally atmospheric.
The majority of the cotton yarns and finer wool yarns are dyed under pressure in what is known as package dye machinery. The yarn is machine wound onto a perforated dye tube. The resulting mass of yarn is known as a package. In production, many of these packages are stacked onto center cores and placed into a pressurized vessel. The dye is forced through the cores, dyeing from the inside of the package to the outside of the package. Then the flow is reversed and the dye moves from the outside to the inside. Package dyed yarns are used to produce both woven and knitted products. The dyed yarn can be easily wound onto cones that the industry uses to set up both weaving and knitting equipment. Yarns can also be beam dyed. Beam dyeing is done in the same manner as package dyeing. A beam is simply a gigantic package. Package and beam dyeing broadly fit in the pressurized batch process category of dye methods.
Yarns can also be dyed on a continuous range. This method is mostly used for producing denim products. Special continuous ranges are used for indigo dye methods. These indigo ranges allow bundles or "ropes" of continuous yarn to pass through a series of troughs or "boxes" containing the dye. Between each successive "dip" in the dye box, the yarn travels through a series of rollers or "pads" that squeeze the excess dye off, and then the yarn travels up into the air before continuing on into the next box of dye. This is known as "dipping skying". The ropes of yarn come off a beam at the beginning of the indigo range and are taken up on another beam at the end of the range. This method of dyeing is not a batch process, rather it is a continuous process.
Garment dyeing involves the dyeing of completed garments or other sewn products. The products are cut and sewn using undyed fabric and then dyed in one of two basic types of garment dye machines.
Some garment dyeing is done in what is known as paddle equipment. Paddle equipment is simply a heated vessel with a paddle wheel attached to the top. In order to operate, the vessel must be completely full so that the paddle moves the surface of the dyebath, thus circulating the garments in the dyebatch. Soaking wet garments must be removed from the vessel and transferred to an extractor for water removal prior to drying.
Rotary garment dye equipment resembles front-loading washers. These machines are water-tight machines that circulate the garments via a rotating drum. Most rotary garment dye machines also extract liquid from the garments using high-speed rotation of the drum.
There are many textile operations that dye the fabric after it has been woven or knitted. The various processes used to dye fabric are known generically as "piece" dyeing. Here again, there are batch piece-dye processes and continuous piece-dye processes.
Most woven materials are dyed on a continuous range. The undyed fabric comes off of a roll and is held taut in its full open-width position as it passes through one or two dye boxes, then through rollers or pads that help distribute and squeeze the dye into the fabric. The fabric then enters a steam cabinet where heat is applied and the dye is set. It then enters a drying chamber and comes out of the dryer as a dyed fabric.
Knit fabrics are mostly dyed in "rope" form, meaning that the fabric is not held taut in an open-width position. The fabric is allowed to bunch up and forms what is known as a rope. This method of dyeing is also called "piece dyeing." The primary methods used for piece dyeing knit fabrics are jet-dyeing and beck-dyeing. Jet dye equipment is pressurized. Beck dye equipment is not. In all cases, the fabric is made to circulate through the dye bath. Several rolls of knit fabric are sewn together end-to-end to form a huge loop of knit fabric. This loop of fabric is fed into the machines. Jet dye machines circulate the fabric using the pressurized flow of the dyebath itself to move the fabric. Beck dye machines use a mechanical system to pull the fabric out of the dye bath in the front of the machine over rotating arms and back down into the dye bath at the back of the machine. Jet dye machines typically use less water and are airtight machines. Hybrid machines that are similar to both jet and beck dyeing are also used.
In a commercial piece dyeing machine, the fabric to be dyed is generally in the form of a seamed continuous loop of fabric, although the ends may be loose in certain machines. The width of the fabric may, for example, be 28 to 60 inches and 50 to 1,000 pounds or more of fabric may be dyed in each batch. The fabric is continuously lifted or circulated out of the dye liquor.
In conventional dye processes and equipment, the fibers being dyed, the dye bath, and the surrounding atmosphere all contain oxygen which would interfere with indigo dyeing, causing oxidation and waste of large quantities of indigo. As oxidized indigo is insoluble, it would form a polluting scum and not be taken up into the fibers. The dyed product would be streaked and have deposits of oxidized indigo on its surface. Large quantities of reducing agents in the dye bath would be required and would be consumed by the oxygen present in the system.
No conventional commercial indigo dye apparatus allows for input into a substantially closed container of undyed fabric and leuco indigo and output from said container of dyed fabric; and no conventional commercial indigo dye process utilizes nitrogen or other inert or non-oxidizing gas to reduce or eliminate the need for conventional reducing agents.
It has not previously been possible to dye with indigo in conventional garment dye machines wherein the constant agitation of the fabric causes excessive oxidation of indigo in the dye liquor resulting in little or uneven uptake by the fabric and loss of most of the dye to the insoluble oxidized form. Similarly, conventional piece dyeing machines, range dyeing and package dyeing machines involve significant contact of the fabric being dyed with air with concomitant oxidation of the indigo.
There is thus a need for commercial processes and apparatuses for dyeing fibers, particularly cellulose fibers, which are suitable for natural and synthetic indigo and which produce a consistent natural, washfast and lightfast color.